This invention relates to secure communication systems and particularly to such systems using a spread spectrum.
Methods of phase modulating a carrier using pseudo noise generators have been used to provide secure communications. Basically, such systems employ means to continuously vary the phase of a data modulated carrier in accordance with a pseudo random code, thereby expanding or spreading the spectrum of the carrier so that an intercept system sees only a noiselike spectrum. A receiver having knowledge of the pseudo random code can easily extract the data. In addition, an intercept system not having knowledge of the pseudo random code can use a square law or higher order detector to collapse the intercepted signal into a second harmonic carrier, thus exposing the fact that a carrier has been transmitted and the basic carrier frequency. Thereafter, the data can possibly be extracted illicitly using known decoding techniques.